Fluid compositions which undergo a change in apparent viscosity in the presence of a magnetic field are referred to as Bingham magnetic fluids or magnetorheological materials. Magnetorheological materials normally are comprised of ferromagnetic or paramagnetic particles, typically greater than 0.1 micrometers in diameter, dispersed within a carrier fluid and in the presence of a magnetic field, the particles become polarized and are thereby organized into chains of particles within the fluid. The chains of particles act to increase the apparent viscosity or flow resistance of the overall fluid and in the absence of a magnetic field, the particles return to an unorganized or free state and the apparent viscosity or flow resistance of the overall material is correspondingly reduced. These Bingham magnetic fluid compositions exhibit controllable behavior similar to that commonly observed for electrorheological materials, which are responsive to an electric field instead of a magnetic field.
Both electrorheological and magnetorheological materials are useful in providing varying damping forces within devices, such as dampers, shock absorbers and elastomeric mounts, as well as in controlling torque and or pressure levels in various clutch, brake and valve devices. Magnetorheological materials inherently offer several advantages over electrorheological materials in these applications. Magnetorheological fluids exhibit higher yield strengths than electrorheological materials and are, therefore, capable of generating greater damping forces. Furthermore, magnetorheological materials are activated by magnetic fields which are easily produced by simple, low voltage electromagnetic coils as compared to the expensive high voltage power supplies required to effectively operate electrorheological materials. A more specific description of the type of devices in which magnetorheological materials can be effectively utilized is provided in copending U.S. patent application Ser. Nos. 07/900,571 and 07/900,567 entitled "Magnetorheological Fluid Dampers" and "Magnetorheological Fluid Devices," respectively, both filed Jun. 18, 1992, the entire contents of which are incorporated herein by reference.
Magnetorheological or Bingham magnetic fluids are distinguishable from colloidal magnetic fluids or ferrofluids. In colloidal magnetic fluids the particles are typically 5 to 10 nanometers in diameter. Upon the application of a magnetic field, a colloidal ferrofluid does not exhibit particle structuring or the development of a resistance to flow. Instead, colloidal magnetic fluids experience a body force on the entire material that is proportional to the magnetic field gradient. This force causes the entire colloidal ferrofluid to be attracted to regions of high magnetic field strength.
Magnetorheological fluids and corresponding devices have been discussed in various patents and publications. For example, U.S. Pat. No. 2,575,360 provides a description of an electromechanically controllable torque-applying device that uses a magnetorheological material to provide a drive connection between two independently rotating components, such as those found in clutches and brakes. A fluid composition satisfactory for this application is stated to consist of 50% by volume of a soft iron dust, commonly referred to as "carbonyl iron powder," dispersed in a suitable liquid medium such as a light lubricating oil.
Another apparatus capable of controlling the slippage between moving parts through the use of magnetic or electric fields is disclosed in U.S. Pat. No. 2,661,825. The space between the moveable parts is filled with a field responsive medium. The development of a magnetic or electric field flux through this medium results in control of resulting slippage. A fluid responsive to the application of a magnetic field is described to contain carbonyl iron powder and light weight mineral oil.
U.S. Pat. No. 2,886,151 describes force transmitting devices, such as clutches and brakes, that utilize a fluid film coupling responsive to either electric or magnetic fields. An example of a magnetic field responsive fluid is disclosed to contain reduced iron oxide powder and a lubricant grade oil having a viscosity of from 2 to 20 centipoises at 25.degree. C.
The construction of valves useful for controlling the flow of magnetorheological fluids is described in U.S. Pat. Nos. 2,670,749 and 3,010,471. The magnetic fluids applicable for utilization in the disclosed valve designs include ferromagnetic, paramagnetic and diamagnetic materials. A specific magnetic fluid composition specified in U.S. Pat. No. 3,010,471 consists of a suspension of carbonyl iron in a light weight hydrocarbon oil. Magnetic fluid mixtures useful in U.S. Pat. No. 2,670,749 are described to consist of a carbonyl iron powder dispersed in either a silicone oil or a chlorinated or fluorinated suspension fluid.
Various magnetorheological material mixtures are disclosed in U.S. Pat. No. 2,667,237. The mixture is defined as a dispersion of small paramagnetic or ferromagnetic particles in either a liquid, coolant, antioxidant gas or a semi-solid grease. A preferred composition for a magnetorheological material consists of iron powder and light machine oil. A specifically preferred magnetic powder is stated to be carbonyl iron powder with an average particle size of 8 micrometers. Other possible carrier components include kerosene, grease, and silicone oil.
U.S. Pat. No. 4,992,190 discloses a rheological material that is responsive to a magnetic field. The composition of this material is disclosed to be magnetizable particles and silica gel dispersed in a liquid carrier vehicle. The magnetizable particles can be powdered magnetite or carbonyl iron powders with insulated reduced carbonyl iron powder, such as that manufactured by GAF Corporation, being specifically preferred. The liquid carrier vehicle is described as having a viscosity in the range of 1 to 1000 centipoises at 100.degree. F. Specific examples of suitable vehicles include Conoco LVT oil, kerosene, light paraffin oil, mineral oil, and silicone oil. A preferred carrier vehicle is silicone oil having a viscosity in the range of about 10 to 1000 centipoise at 100.degree. F.
Many magnetorheological materials such as those described above suffer from excessive gravitational particle settling which can interfere with the magnetorheological activity of the material due to non-uniform particle distribution. One cause of gravitational particle settling in magnetorheological materials is the large difference between the specific gravity of the magnetic particles (e.g., iron=7.86 gm/cm.sup.3) and that of the carrier fluid (e.g., silicone oil=0.95 gm/cm.sup.3) which can cause rapid particle settling in a magnetorheological material. The metallic soap-type surfactants (e.g., lithium stearate, aluminum distearate) traditionally utilized to guard against particle settling inherently contain significant amounts of water which can limit the useful temperature range of the overall magnetorheological material. The use of a silica gel dispersant as disclosed in U.S. Pat. No. 4,992,190 has presently been found not to significantly minimize particle settling over a prolonged period of time.
A need therefore currently exists for a magnetorheological material that exhibits minimal particle settling for a prolonged period of time and that can be utilized over a broad temperature range.